This invention relates to a single mode optical fiber. Various technical developments have been made with recent development in the art of optical communication. In recent years, in addition to various advantages of optical communication, for example, light weight, not inductive and no cross-talking it became possible to transmit data at a lower loss and over a wider frequency band than existing transmission lines utilizing such metal conductors as coaxial cables or millimeter waveguides. Recently, a long distance transmission over a distance of 100 Km at a frequency of 1 GHz is planed with optical fibers without using any repeating station and over ultra wide frequency band. To realize such a long distance ultra wide frequency band transmission it has been theoretically determined that the total dispersion of light should be less than .+-.1 ps /km/nm and that the transmission loss should be less than 0.5 dB/km.
Considering the transmission characteristics of the present day optical fiber from the stand point described above, there are caused certain problems for the reasons as will be described hereunder for effecting a ultra wide frequency band long distance transmission.
More particularly, light incident into the core of an optical fiber is propagated along the core by repeating reflections at the interface between the core and the cladding. The degradation of the transmission characteristics caused by the dispersion of light can be summarized into the following three items.
(1) multimode dispersion
This phenomenon is caused by the fact that the transmission constant representing the transmission conditions vary nonlinearly with respect to the angular frequency of light with the result that as the angular frequency increases higher order modes appear in the light propagating mode so that the group velocity differs for different modes
(2) material dispersion (.sigma..sub.M)
This phenomenon is caused by the fact that the refractive index of glass constituting the optical fiber varies nonlinearly with respect to the wavelength of light, and the material dispersion .sigma..sub.M of a single mode optical fiber is shown by the following equation ##EQU2## where nl represents the refractive indix of the core, .lambda. the wavelength of light, and C the velocity of light in vacuum.
(3) Waveguide dispersion (.sigma..sub.W)
This dispersion is determined by a relation between the propagation constant .beta. and the angular frequency .omega. of light and in the case of the single mode optical fiber, the waveguide dispersion .sigma..sub.W is given by the following equation ##EQU3##
Regarding a multimode optical fiber above described three types of dispersions should be considered but in this invention, for the purpose of minimizing the dispersions so as to decrease signal distortions only a single mode optical fiber is considered in which case the dispersion (1) is not necessary to be taken into consideration and only dispersions (2) and (3) should be taken into consideration. Accordingly, for the single mode optical fiber the sum (.sigma..sub.M +.sigma..sub.W) of the material dispersion .sigma..sub.M and the waveguide dispersion .sigma..sub.W corresponds to the total dispersion .sigma..sub.T and the modulated frequency band width f of light that can transmit the signal without distortion is given by the following equation ##EQU4##
A typical prior art optical fiber has a core diameter 2a=9.0 microns, the refractive index of core nl=1.46319, the difference between the refractive indices of the cladding and core ##EQU5## Such data are shown in a K. Daikoku et al paper entitled "Direct measurement of wavelength dispersion in optical fibers-difference method", Electronics Letters, 1978, Vol. 14. No. 5 pages 149-151.
With such prior art optical fiber, as the wavelength of light is varied from 0.9 to 1.6 microns, the material dispersion .sigma..sub.M varies in a range of from +66 ps/km/nm to -25 ps/km/nm, and in a case wherein the refractive index difference and the core diameter have values just described the waveguide dispersion varies in a range of from 4 to 13 ps/km/nm. For this reason, the total dispersion .sigma..sub.T varies in the positive and negative directions about .lambda.=1.43 microns. Accordingly, where the optical fiber is used at a wavelength .lambda.=1.43 microns at which the total dispersion is zero the distortion of the signal becomes a minimum thus enabling long distance wideband transmission. However, slight deviation of the wavelength from .lambda.=1.43 microns, causes the total dispersion .sigma..sub.T to rapidly increase whereby the frequency bandwidth f given by equation (1) would be rapidly narrowed.
Accordingly, with the prior art optical fiber even when the total dispersion .sigma..sub.T is selected to be .+-.1 ps/km/nm necessary for long distance transmission as has been pointed out before, the usable wavelength width is at more 0.02 microns and the wavelength is 1.41-1.43 microns. For this reason, for transmitting signals over a long distance with a wide frequency band efficient transmission is not possible even when the wavelength is multiplexed by wavelength division technique for the purpose of improving the transmission efficiency.